We have developed GC-MS methodology by which to quantitate 9-HODE (hydroxyoctadienoic acid) and 13-HODE in the rodent mammary gland and found high levels of 9-HODE and 13-HODE in the mammary glands of mice consuming a diet high in corn oil (20% by weight) (a diet that is highly mammary tumorigenic), low levels in mice consuming a diet low in corn oil (5%) (a diet that is poorly mammary tumorigenic) and low levels in glands of mice consuming beef tallow (20%) (a diet that is also poorly mammary tumorigenic). The increased levels of 9-HODE and 13-HODE in the mammary glands of these animals could be a result of enzymatic processes (lipoxygenase) and/or non-enzymatic (free radical) processes. How these oxidative products of linoleic acid might stimulate cell proliferation processes is not known.